During postnatal development, DIAm fibers display dramatic growth that is disproportionate across fibers expressing different myosin heavy chain (MHC) isoforms. Postnatal increase in specific force of DIAm fibers also varies across fibers expressing different MHC isoforms. Unilateral denervation (DNV) dramatically retards postnatal growth of DIAm fibers and reduces specific force. These data suggest that nerve-derived factors (such as the family of neurotrophins) play a key role in postnatal muscle fiber growth. We hypothesize that neurotrophins modulate the postnatal growth of DIAm fibers and MHC protein expression and thereby affect specific force. The major goals of the proposed studies are: 1) to explore the mechanisms regulating MHC protein expression during postnatal growth of DIAm fibers, 2) to explore the effects of TrkB receptor activation on MHC protein expression, 3) to explore the effects of DNV (i.e., removal of neurotrophin influence) on MHC protein expression, and 4) to examine the relationship between postnatal changes in MHC protein content and changes in specific force generated by DIAm fibers. There are four specific aims in the proposed studies: Specific Aim #1: To examine the impact of DNV on postnatal changes in myonuclear domain size in DIAm fibers Hypothesis la - Following DNV, myonuclear domain size decreases and this effect varies with postnatal age and across different MHC isoforms. Hypothesis lb - In cultured myotubes/myofibers, TrkB receptor activation increases myonuclear domain size. Specific Aim #2: To evaluate the impact of DNV on postnatal changes in MHC isoform transcription in DIAm fibers. Hypothesis 2a - Following DNV, MHC transcription decreases and this effect varies with postnatal age and across different MHC isoforms. Hypothesis 2b - In cultured myotubes/myofibers, TrkB receptor activation increases MHC transcription. Specific Aim #3: To evaluate the impact of DNV on postnatal changes in MHC protein content per haft sarcomere and MHC protein turnover in DIAm fibers. Hypothesis 3a - Following DNV, MHC protein content per half sarcomere decreases and this effect varies with postnatal age and across different MHC isoforms. Hypothesis 3b - Following DNV, the fractional synthesis rate of MHC decreases while degradation rate increases, and these effects on MHC turnover vary with postnatal age and across different MHC isoforms. Hypothesis 3c - In cultured myotubes/myofibers, TrkB receptor activation increases MHC protein expression. Specific Aim #4: To evaluate the impact of DNV on postnatal changes in specific force of DIAm fibers. Hypothesis 4a - Following DNV, specific force of DIAm fibers decreases and this effect varies with postnatal age and across fibers expressing different MHC isoforms. Hypothesis 4b - Following DNV, the decrease in DIAm fiber specific force reflects both a decrease in MHC content per half sarcomere and a decrease in the force per cross bridge. [unreadable] [unreadable]